The present invention relates to an automatic loading amount control apparatus for a granular compound fertilizer material, which optimally controls a water content, having a large correlation with a grain size, of a loading material when a granular compound fertilizer, e.g., a nitrogenous fertilizer, having a grain size of about several mm is manufactured by a rotary granulator.
Generally, the grain size of a granular compound fertilizer obtained from a rotary granulator is related to the water content of the material loaded in the rotary granulator. Generally speaking, when the water content is increased, the grain size is increased; when the water content is decreased, the grain size is decreased.
Hence, in the manufacture, the water content in a material loaded in the rotary granulator is conventionally controlled in the following manner so that grains having a size falling within the allowable range of the product size are obtained from the rotary granulator at a high proportion, i.e., so that a granular compound fertilizer having a product size can be manufactured at a high yield.
The weight-converted grain size distribution of the granular compound fertilizer obtained from the rotary granulator is measured. The weight proportions of grains having a size falling within the product size range, grains having a size of a range higher than the product size range, and grains having a size of a range lower than the product size range are calculated from the weight-converted grain size distribution. Weight proportions of the same ranges are obtained in advance by a hand sieve of the operator when ideal granulation is performed. The calculated proportions are compared with the ideal proportions. Then, whether or not the size of the grains to be formed must be entirely increased, and the like are recognized. The water content in the material is adjusted in accordance with the recognition result. An example of this technique is Japanese Patent Laid-Open No. 1-275490.
Conventionally, the yield of the manufacture is increased by the arrangement as described above. However, the following problems remain to be solved.
Since the proportion of the ideal grain size as the target value of control is fixed, if the ideal target value itself includes a measurement error, the yield of the manufacture is decreased.
The distribution pattern of the proportions of the respective grain size ranges appearing when a high yield is obtained in a rotary granulator is not limited to one type, but changes more or less from time to time, although the reason for this has not yet been clarified. More specifically, sometimes the proportion of grains having a size falling within the product size range is 60%, the proportion of grains having a size of a range higher than the product size range is 20%, and the proportion of grains having a size of a range lower than the product size range is 20%, thus providing a maximum yield of 60%; sometimes the proportions change like 60%, 5%, and 35% in the same order, thus providing a maximum yield of 60%. Even if the proportion of the ideal grain size as the target control value is correctly measured, this proportion holds only in this specific case. For this reason, when the current proportion is different from that of the ideal value, since control is performed with reference to the ideal value, that cannot be currently obtained, as the target value, the yield fluctuates largely, and an average yield is not actually increased.
Furthermore, regarding the comparison of the ideal and actually measured proportions of the grains having a size falling within the product size range, the grains having a size of a range higher than the product size range, and the grains having a size of a range lower than the product size range, since comparison of complex combinations is possible, the comparing methods become complex, thus making control very difficult.